A mixer circuit serves as a downconverter for the frequency conversion of a radiofrequency signal (radiofrequency=RF) into an intermediate frequency signal (intermediate frequency=IF) or as an upconverter in the opposite direction, a local oscillator signal (LO) being used for the frequency conversion. In this case, the frequency fIF of the intermediate frequency signal results from the frequency fRF of the radiofrequency signal and the frequency fLO of the local oscillator signal in accordance with fIF=fRF±fLO. An elementary multiplier circuit may already be used as a simple mixer circuit.
An electronic circuit having a mixer circuit is usually used in a modern mobile radio device, for example in a mobile telephone, which is generally battery-operated. This electronic circuit is provided with a voltage controlled local oscillator, which should have in addition to a maximized low noise, owing to the battery operation, a minimized current consumption and also a low operating voltage. Voltage controlled local oscillators, which are also referred to as VCO (voltage controlled oscillator) are known from the prior art, for example from Cranickx J., Steyaert M. S. J.: “A 1.8 GHz low-phase noise CMOS VCO using optimized hollow spiral inductors” in IEEE J. of Solid-State Circuits, vol. 32, No. 5, pp. 736-744 (1997), Zannoth M., Kolb B., Fenk J., Weigel R.: “A Fully Integrated VCO at 2 GHZ” in IEEE J. of Solid-State Circuits, vol. 33, No. 12, pp. 1987-1991 (1998), and Tiebout M.: “Low-Power Low-Phase-Noises Differentially Tuned Quadrature VCO Design in Standard CMOSI” in IEEE J. of Solid-State Circuits, vol. 36, No. 7, pp. 1018-1024 (2001).
The mixer circuit contains a nonlinear component for the frequency conversion. Diodes or transistors may be used as nonlinear components.
A transistor mixer is generally operated actively in order to achieve a maximum conversion gain of the mixer circuit by means of the amplification of the transistor. A transistor mixer can also be operated passively, the transistor channel then being used as a variable resistor.
Among silicon components, at the present time bipolar transistors are preferably used in mixer applications on account of their nonlinear current-voltage characteristic curve having an exponential profile. In the case of a mixer circuit having field effect transistors based on metal oxide semiconductors (MOSFET=metal-oxide semiconductor field effect transistor), nonlinear current-voltage characteristic curves that are characterized by a quadratic function result, as is described in Meinke H. H., Grundlach F. W.: “Taschenbuch der Hochfrequenztechnik” [“Pocket book of radiofrequency technology”] Springer Verlag, 5th edition, volume 1, pp. G18-G21 (1992).
The simplest form of a mixer circuit is the differential amplifier, which multiplies only in two quadrants of the current-voltage characteristic curves. Mixer circuits are generally constructed symmetrically, however, in order to be able to multiply in all four quadrants of the current-voltage characteristic curves. A more exact signal conversion is thereby achieved.
Mixer circuits which are produced by means of modern semiconductor technology and have CMOS transistors (CMOS=complementary metal-oxide semiconductor), for example, have a reduced breakdown voltage at high frequencies to be converted. Consequently, the maximum permissible operating voltage for the mixer circuit is reduced and, in accordance with the prior art, is usually only between 0.9 V and 1.5 V. Since every transistor has an internal resistance, a certain voltage ΔU is dropped across a transistor when current flows through the latter. In order to enable a current flow through the transistors when a plurality of transistors are connected together in series, the operating voltage present across all the transistors must therefore be greater than the sum of the voltages ΔU dropped across the transistors. However, if all the other transistors are in the on state apart from a single transistor in the off state, the entire operating voltage is present across the transistor in the off state. Therefore, the operating voltage should be chosen to be less than the lowest breakdown voltage of the transistors in order to avoid a voltage breakdown in the transistor in the off state. Consequently, the breakdown voltage of the transistors limits the permissible magnitude of the operating voltage applied across all the transistors. This furthermore has the consequence that the number of transistors is limited in the design of an integrated circuit having a plurality of transistors connected in series. Consequently, the number of stackable transistor planes in an integrated circuit, i.e. the number of transistors connected one after the other in series, is limited, in which case it should be noted that there may be a plurality of transistors present in different current paths at the same potential level.
The prior art discloses mixer circuits in which the number of transistor planes is reduced, but not the total number of transistors in the mixer circuit. Such mixer circuits are known by the designation “current-folded” mixer circuit. In the case of “current-folded” mixer circuits, the mixer circuit is divided into a plurality of subcircuits that are electrically coupled to one another in a suitable manner such that said subcircuits in each case have only a few, for example two, transistor planes, as a result of which the operating voltage of the mixer circuit can be reduced. The subcircuits that are electrically coupled to one another in a suitable manner ensure the desired signal flow.
FIG. 2 illustrates a known “current-folded” mixer circuit 200 in accordance with the prior art, which enables the use of more transistors than transistor planes provided despite a low operating voltage. The “current-folded” mixer circuit 200 illustrated in FIG. 2 is a four-quadrant mixer circuit, which is also known by the designation “current-folded double-balanced mixer”. In this case, the description “double-balanced” indicates that the local oscillator signal is coupled in uniformly at a plurality of locations in the mixer circuit.
In the “current-folded” mixer circuit 200, a radiofrequency signal RF+ is coupled in via the base of a first npn transistor T1 and the inverted radiofrequency signal RF− with respect to the radiofrequency signal RF+ is coupled in via the base of a second npn transistor T2. The emitters of the two npn transistors T1, T2 are electrically coupled to one another by means of an electrical resistor R1. The emitter of the npn transistor T1 and T2 is electrically coupled to the collector of a controlling third and fourth npn transistor T3 and T4, respectively. A constant control voltage BIAS is present at the base of the controlling third and fourth npn transistor T3 and T4, respectively, and the collector of the controlling third and fourth npn transistor T3 and T4, respectively, is electrically coupled to the ground potential via an electrical resistor R2 and R3, respectively. The collectors of the two npn transistors T1, T2 are electrically coupled to the operating voltage VCC by means of an electrical resistor R4 and R5, respectively.
In addition, the base of a fifth npn transistor T5 is electrically coupled to a first node KN1 between the electrical resistor R4 and the collector of the first npn transistor T1, the collector of said fifth npn transistor likewise being electrically coupled to the operating voltage VCC. Analogously to this, the base of a sixth npn transistor T5 is electrically coupled to a second node KN2 between the electrical resistor R5 and the collector of the second npn transistor T2, the collector of said sixth npn transistor in turn being electrically coupled to the operating voltage VCC. The emitter of the fifth npn transistor T5 is electrically coupled to the collector and, in parallel therewith, to the base of a seventh npn transistor T7 and also to the base of an eighth npn transistor T8. The emitters of the two npn transistors T7, T8 are respectively electrically coupled to the ground potential via an electrical resistor R6, R7. In a comparable manner, the emitter of the sixth npn transistor T6 is electrically coupled to the collector and, in parallel therewith, to the base of a ninth npn transistor T9 and also to the base of a tenth npn transistor T10. The emitters of the two npn transistors T9, T10 are respectively electrically coupled to the ground potential via an electrical resistor R8, R9.
The emitters of two npn transistors T11, T12 and T13, T14 are electrically coupled to the collector of the eighth and tenth npn transistors T8 and T10, respectively. The local oscillator signal LO+ is present at the bases of the two npn transistors T11, T13 and the inverted local oscillator signal LO− with respect to the local oscillator signal LO+ is present at the bases of the two npn transistors T12, T14. The two npn transistors T11, T13 and T12, T14 are electrically coupled to one another by their collectors and, via a respective electrical resistor R10 and R11, to the operating voltage VCC. The intermediate frequency signal IF+ can then be tapped off at a third node KN3 between the collectors of the two npn transistors T12, T13, on the one hand, and the electrical resistor R10, on the other hand, while the inverted intermediate frequency IF− with respect to the intermediate frequency signal IF+ can be tapped off at a fourth node KN4 between the collectors of the two npn transistors T11, T14, on the one hand, and the electrical resistor R11, on the other hand.
Clearly, the “current-folded” mixer circuit 200 constitutes a horizontally constructed Gilbert mixer. However, the “current-folded” mixer circuit 200 has the disadvantage that the current mirrors within the “current-folded” mixer circuit 200 are generally slow. This means that a variation of the radiofrequency signal RF+ (and thus also of the inverted radiofrequency signal RF−) causes a temporally delayed change in the intermediate frequency signal IF+ (and also in the inverted intermediate frequency signal IF−). Consequently, on account of the temporal delay in the “current-folded” mixer circuit 200, the intermediate frequency signal, in the event of conversion from a radiofrequency signal, may have a frequency which is considerably reduced in comparison with the intermediate frequency that is actually to be generated. Since this is undesirable, the “current-folded” mixer circuit 200 can only be operated with low operating frequencies if reliable signal conversion and signal transmission are to be effected.